The present invention relates to peening of assembled rotor parts while still in the casing or unit rotor and, more particularly, to ultrasonic peening treatment of rotor components for gas turbines, steam turbines or hydro machines wherever shot peening is deemed necessary or desirable.
It is generally recognized that fatigue life for certain materials is enhanced when parts are shot peened. Peening induces a residual compressive stress that retards crack initiation. The most widely used peening technology involves a large quantity (many pounds) of small sized metallic or ceramic ‘shot,’ which is propelled at the component to be peened. In a shop environment, the small shot can be fairly easily cleaned from the component to avoid having it introduced into a working turbine.
In instances where rotor parts are repaired or modified in the field, parts may be required to be re-shot peened in order to introduce the compressive stress to resist cracking upon return to service. In a field application, however, a conventional shot peen process scatters shot widely about the work area, and the small pieces of shot are not easily retrieved from the turbine unit. Residual shot in the unit poses a threat to the operation of the turbine.
Forms of peening other than conventional exist, such as laser shock, water cavitation shock, and the like; however, these forms are either very expensive or not readily field adaptable.
Ultrasonic peening is a commercially available technology that generally uses a fixed computer-controlled machine in a shop environment to peen components of a fixed shape. This configuration generally requires either (1) the components to be a maximum size (such as a piece part), or (2) the machinery to be large scale in order to treat the component as specified. Existing applications of peening on rotor components typically perform the operation with separate pieces comprising the rotor rotating to the peening equipment or the peening equipment manipulating around the separate parts in a horizontal plane. As a consequence, the existing applications are not suitable for use in situ. Additionally, the existing applications lack equipment mobility and are typically unable to operate on a vertically rotating component.
Additionally, other shot peening methods (e.g., conventional, water jet cavitation, laser) require a “line-of-sight” such that the media doing the peening (metal or ceramic shot, water jet, laser beam) must be in line with the object to be peened or be able to ricochet and peen the surface of interest therein. With most processes, a line-of-sight is not available while the rotor is still in the casing and/or the rotor assembly is still intact.
An additional concern with conventional shot peening is that some of the shot would remain in the assembled rotor or casing, causing subsequent premature failure of other parts, such as buckets, nozzles or bearings, upon return to service. It would thus be desirable to enable a rotor component to be peened without disassembly and without potential contamination by shot media.